The basic philosophy of relaying is to clear each fault as quickly as possible by removing the minimum portion of the electrical system from service. This provides the maximum overall continuity of service. The speed at which a fault is cleared using relaying is largely dependent upon how much money is invested in the system to do the job. However, technical considerations of the minimum relay operating time also must be considered. The faults which are commonly protected against on transmission lines are short circuits between two or three phases, and ground on one or two phases. Many other types of faults are guarded against in industrial and commercial electrical systems.
